Despite pressure from abortion-rights supporters, the former president of NARAL Pro-Choice America said she would not run in the November election, which will likely feature two candidates who oppose abortion: Democrat Bob Casey Jr. and incumbent Republican Rick Santorum.

Michelman, 63, considered entering the race after Casey said he would have voted to confirm Samuel Alito for the U.S. Supreme Court. There was fear among many Democrats that Michelman’s entrance into the race would split the vote in November in Santorum’s favor.

“Despite profound and fundamental differences, I have decided that Pennsylvania will be better served by electing Bob Casey to the U.S. Senate than giving his opponent another term,” Michelman wrote in an op-ed piece published in Sunday’s edition of The Philadelphia Inquirer. “I do this knowing that I may forever regret not responding one more time to the clarion call of principle.”

A spokesman for Casey said he had no comment on Michelman’s decision not to run.